In recent years, an ultra violet light source that receives a large amount of input electric power has been widely used in a manufacturing process of a to-be-processed product such as semiconductors, liquid crystal substrates, a color filters and the like.
It is a certain type of a high pressure discharge lamp capable of generating an arc discharge between electrodes in an arc tube enclosing mercury vapor or noble gas that is most frequently used as this type of the ultra violet light source.
In the meantime, in the above mentioned manufacturing process, it is required to reduce the processing time more and more. For this reason, the high pressure discharge lamp to be used in the above mentioned application is required to improve the radiance thereof more and more. Thus, it is required to increase the input electric power in order to improve the radiance of the high pressure discharge lamp.
Nevertheless, when the input electric power to the lamp is simply increased, it leads to a larger load to the electrodes of the discharge lamp. As a result, it is likely to entail a possible problem in which a lamp becomes blacken and becomes short-lived due to the evaporation of an electron radioactive substance from the electrodes.
In order to cope with the above mentioned problem in the high pressure discharge lamp, a certain technique has been proposed in which an energy from a laser is input into a discharge space to excite a light emitting gas so as to obtain the ultra violet light radiation (or irradiance), which is exemplarily disclosed in the Patent Literature 1 (Laid-open Publication of Japanese Patent Application No. 2010-170122 A).
This type of light source is referred to as a Laser Produced Plasma (LPP) light source, or alternatively, Laser Sustained Plasma (LSP).
According to prior art exemplarily disclosed in the Patent Literature 1 (Laid-open Publication of Japanese Patent Application No. 2010-170122 A), a plasma generating vessel 30 includes a light emitting part 31, which is made from quartz glass, and a sealing part 32. The light emitting part 31 encloses, for example, mercury and xenon gases as a light emitting substance.
In the example of the Patent Literature 1, the plasma generating vessel 30 is implemented as an electrodeless plasma generating vessel. The plasma generating vessel 30 is disposed at one of focal points F1 of an ellipsoidal reflector mirror 40.
On the other hand, a laser beam generator 50 is disposed in front of the ellipsoidal reflector mirror 40. The laser beam generator 50 emits the laser beam of, for example, pulsed laser beam or a Continuous Wave (CW) laser beam and the emitted laser beam is introduced into the plasma generating vessel 30.
The laser beam emitted from the laser beam generator 50 proceeds through a window part 61 of a plane (planar) mirror 60, and condensed by a light condensing lens 70, which is disposed between the window part 61 and the plasma generating vessel 30, such that the plasma generating vessel 30 is irradiated with the condensed beam. By condensing the laser beam, it makes it possible to increase the energy density at the focal point F1. Thus, it makes it possible to excite the light emitting substance so as to generate the radiation (radiated) light. The resulting radiation light from the plasma generating vessel 30 is reflected by the ellipsoidal reflector mirror 40 and further reflected by the plane mirror 60 so as to proceed toward an object to be irradiated.
In the above mentioned conventional LPP (or LSP) lamp, the quartz glass is employed as a material of the plasma generating vessel. However, the plasma generating vessel employing the quartz glass may entail a problem in which the plasma generating vessel tends to undergo the distortion, which is caused by the ultra violet light or the vacuum ultra violet light, because the plasma generating vessel is irradiated with the high power ultra violet light (UV light) and the high power vacuum ultra violet light (VUV light) from the plasma.
As the above mentioned distortion caused by the ultra violet light or the vacuum ultra violet light accumulates, the accumulated distortion caused by the ultra violet light or the vacuum ultra violet light may then cause a glass surface to crack. It is concerned that the cracked portion may trigger the lamp to be damaged or even corrupted.
In order to avoid the above mentioned defect, by employing, for the plasma generating vessel, a crystalline material such as crystal (crystallized quartz) or sapphire instead, it may be possible to reduce the distortion caused by the ultra violet light or the vacuum ultra violet light. However, it would be extremely difficult in terms of manufacturing to mold the crystalline material into a vessel having a cylindrical or spherical shape. Because of the difficulty in the manufacturing process, therefore, use of the crystalline material is not practical.